Urbanization is a major stressor on ecosystems and organisms, mainly through changes in temperature. Water temperature is a key environmental factor, along with food resources, in determining the development of aquatic insects. Our objectives were to assess how urban water temperatures are affecting the development of organisms, using Chironomus sp. as a model organism. Our study system was the Rio Piedras watershed, a highly urbanized system in the San Juan Metropolitan Area, Puerto Rico. We collected temperature data from 6 tributaries within the urbanized watershed creating a gradient from most to least urbanized and a non urban stream as a reference. Temperature was measured every hour using HOBO sensors, one sensor placed in the water and one outside, under riparian vegetation. To assess water temperature effects on development, we set up four treatments: 22°C, 25°C, 30°C and a fluctuating temperature between 24°C and 30°C. We placed the organisms under the treatments, starting with egg masses and allowing them to complete their life cycle up to emergence. We measured egg hatching efficiency, larval size, and time of adult emergence. Our non-urban stream had maximum water temperatures of 23°C and daily diel cycle of 2 degrees. The most urbanized stream reached 30°C and had a daily diel cycle of 6 degrees. We found water and air temperatures significantly related with the level of urbanization in the subwatershed. Water temperature had no effect on egg mass hatching efficiency. In contrast, the number of days to reach instar IV, pupae, and adult emergence decreased as temperature increased. Time to reach instar IV decreased by 44%, pupae 58% and adult emergence 59% in the highest temperature. The treatment with the highest survival to adulthood was the fluctuating temperature treatment. Urbanization significantly increases water temperature in urban streams in Puerto Rico, affecting aquatic insect development.